This invention relates to a line-powered modem or other data handling apparatus including an inductance simulating circuit for exhibiting a relatively low resistance to direct current flow therethrough and a relatively high impedance to alternating signals applied thereto, without the use of an inductive component.
Modems are well known for use as interfaces between digital data equipment and transmission lines, such as telephone lines. They turn input data into frequency shift keyed (FSK) signals comprising tones which are within the frequency domain of the transmission line, and conversely translate FSK signals, received from the transmission line, into digital data.
The advent of the microcomputer has led to a considerable increase in the number of modems required. Such microcomputers are now widely used, for example as home computer equipment, in process control and in the monitoring of operating parameters in industrial plant, and communication over a telephone line, or other transmission line, to and from a remote location is frequently required. The apparatus connected to the line may be operating in a hazardous atmosphere or location, making it essential that all powered circuits are intrinsically safe, i.e. under any normal or faulty operating condition they cannot produce a spark of sufficient energy to ignite the atmosphere.
However, modems are currently too complicated and expensive, and suffer from a number of other disadvantages. Firstly the standard signal formal used for communication requires a signal of .+-.12 volts, which is not available in a home computer, because the power supply is normally only 5 volts d.c. Secondly, a modem (and other equipment) which is to be connected to a telephone line is required by the telephone system operating authorities in many countries to be approved to ensure that it cannot apply dangerous voltages or spurious signals to the telephone system. This involves expensive type-approval tests and subsequent closely controlled manufacture. Thirdly, a power supply is normally required to power the modem, and this will greatly increase the size of the modem structure. Furthermore, the modem power unit must be intrinsically safe if the equipment is to be used under hazardous conditions.
Fourthly, if a loop current has to circulate in the line and in the modem to hold in a line selector relay, then two conflicting requirements have to be met, namely that the modem input circuit must have a low d.c. resistance to allow circulation of the current, and must also have a high a.c. impedance, preferably matched to that presented by the line, to allow reception of tones from the line. A transformer or other inductive component can fulfil both of these requirements, but such components are large, and therefore difficult to accommodate, and also hamper the provision of a single integrated circuit to perform all of the modem functions. Furthermore, such a component may store an appreciable quantity of energy, thereby creating a potential danger.